Drugs taken by humans and animals find their way into rivers, lakes and even drinking water, and can have devastating effects on the environment.

There are three main ways that pharmaceuticals make their way into the environment. By far the biggest contribution comes from drugs taken by people or animals that are then excreted in urine or faeces.

“A good proportion of any drug is excreted,” says Lyons — between 30% and 90% of the active ingredient in an oral dose. And the metabolites of many drugs can also remain active in the environment after being excreted.

The improper disposal of drugs also makes a contribution — when people fail to complete a prescription or clean out their medicine cabinet and throw the leftover drugs in the sink or down the toilet. In both cases, drugs end up in sewage treatment plants, which were generally not designed to remove such pollutants from wastewater. Depending on the drug, removal efficiencies range from 20% to more than 80%.

Some drug manufacturing facilities have also been shown to release active ingredients into nearby waterways, creating localised hotspots of pharmaceutical pollution.